The basic components of a typical ion mobility spectrometer (IMS) include an ionization source, a drift tube that includes a reaction region, an ion shutter grid, a drift region, and an ion detector. In gas phase analysis, the sample to be analyzed is introduced into the reaction region by an inert carrier gas, ionization of the sample is often accomplished by passing the sample through a reaction region and/or an ionization region. The generated ions are directed toward the drift region by an electric field that is applied to drift rings (sometime referred as guard rings or ion guide) that establish the drift region. A narrow pulse of ions is then injected into, and/or allowed to enter, the drift region via an ion shutter grid. Once in the drift region, ions of the sample are separated based upon their ion mobilities. The arrival time of the ions at a detector is an indication of ion mobility, which can be related to ion mass. However, one skilled in the art appreciates that ion mobility is not only related to ion mass, but rather is fundamentally related to the ion-drift gas interaction potential, which is not solely dependent on ion mass.
State-of-the art ion mobility spectrometers include drift tubes with complicated mechanic parts. Each component in the drift tube typically requires the assembly of multiple parts. Such complex mechanical designs significantly increase the cost of ion mobility spectrometer and can also limit the performance of the ion mobility spectrometer. In general, the more parts in the drift tube design, then the higher probability that the drift tube will have technical problems, such as gas leakage, inadequate temperature control, inadequate pressure control, thermal and/or electrical insulation leakage.
For many applications, such as explosive detection in highly contaminated field environments, normal operation of the spectrometer is frequently prevented by overloading the system with large samples or contaminants. Rapid self cleaning mechanisms are highly desired for these applications. Conventional ion mobility spectrometer drift tube constructions are described by Ching Wu, et al., “Construction and Characterization of a High-Flow, High-Resolution Ion Mobility Spectrometer for Detection of Explosives after Personnel Portal Sampling” Talanta, 57, 2002, 123-134. The large thermal mass of the tube structure prevents the system from flash heating and rapid cooling of the ion mobility spectrometric components for cleaning purpose. Spectrometric components can be cleaned by “baking out” the components. However, “baking out” typically takes hours to complete.
Previous publications have indicted that a uniform electric field in the drift region of an ion mobility spectrometer is imperative to achieve high mobility resolution in such devices. See, for example, Ching Wu, et al., “Electrospray Ionization High Resolution Ion Mobility Spectrometry/Mass Spectrometry,” Analytical Chemistry, 70, 1998, 4929-4938. A uniform electric field can be created by reducing the size of each voltage drop step and increasing the number of drift rings. Narrow drift rings are utilized to generate the desired field distribution. However, the more drift rings that are used in a drift tube, the more lead wires are needed to be sealed at the wall to complete the drift tube structure. Structure complication greatly limits the possibility of creating highly uniform electric fields in the drift tube. U.S. Pat. No. 4,712,080 and U.S. Patent Publication No. 2005/0211894 A1 describe layers of conductive coating that are commonly proposed to build the drift tube. However, coatings that are exactly the same thickness along the drift tube are a very challenging to make. Conductive layers with uneven coating thickness will cause distorted electric field distributions and unpredictable system performance.